@Moonrox: Also adding on to what moot said, you don't know how your future before your 20's is going to turn out. Once you get into your 20's things start to change and you begin to experience a lot of things. Vastly different than your years as a Child and Teen. Those 'things' can relate to really anything. Most of them, you'd least expect. Also when you turn 20 and after, you begin to meet and speak with a lot of people, like ones that are from different nationalities. And finally, you'll begin to learn WAY more than you've ever known in your 20's.
If you're going to ask why the "20's", then I'll just tell you right now, that in your 20's you have a lot of opportunity in your life to do something. For Example: In your 20's you may want to go to College to study for a specific subject and travel, or you may want to learn a sport and become a professional player. In your 20's there are many options you can choose from, which is overwhelming for a kid.
In summary, what moot and I are saying is that these are all adult things, and you're a kid. This is the time to be a kid and cherish this time as much as you can. ^_^